In general, waveguides with highly dissipative material are used as absorbers that is matched terminations and as attenuators. The waveguides are provided with a metallic wall surrounding the field in which the dissipative material is arranged in such a manner that a small reflection coefficient is obtained over a wide frequency range and simultaneously, a close heat contact is provided between the dissipative material and the inner wall of the waveguide in order to carry off the heat generated by converting the radio-frequency power through the wall which is suitably cooled at higher power.
The provision of a coating of dissipative material within the field is, however, reduced with increasing frequency of the rf-wave and thus decreasing cross section of the waveguide. Since at increasing frequency also the loss per unit of length of the dissipative material increases and the penetration depth of the rf-wave decreases, considerable power densities are obtained in the area of several gigahertz. In known waveguides, the maximum admissible radio-frequency power at which the temperature of the absorbing material reaches its maximum value, becomes thus steadily smaller with increasing frequency.